enclosure: fix WARN_ON removing an adapter in multi-path devices

We have peculiar problems with multi-path and enclosures: physically, we know
each bay can only be occupied by a single disk device.  However in multi-path,
it appears we have many (because each path to the device appears in Linux as a
different kernel device).  We try to fix this by only having the last seen
device show up in the bay.

Sysfs gets very annoyed if we try to manipulate links when the kobject sysfs
directory (kobj.sd) doesn't exist and drops a huge WARN_ON which most users
panic and report an oops for.  This happens on a few path removal situations
and IBM reports seeing it when one of their multi-path adapters is removed.

Add a check to enclosure device removal for the existence the sysfs directory
containing both the forward and back links so that the remnants (if any) get
removed in either direction but no scary warnings are dumped.

Reported-by: Wen Xiong <wenxiong@linux.vnet.ibm.com>
Tested-by: Wen Xiong <wenxiong@linux.vnet.ibm.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>